The role of metaphors and expressiveness can be observed at multiple levels of granularity. At the level of individual channels there are several examples of how channels may express or fail to express certain types of information. For example, color hue can’t express ordinal or quantitative information because the human eye does not assign an order to colors that vary exclusively in hue. Similarly, colors have strong semantic associations, therefore appropriate associations between concepts and colors may improve readability and comprehension [1]}. Area size is a moderately precise channel when conveying quantity, but it cannot easily show negative values because larger sizes are firmly associated with larger (positive) quantities.
